Particle-measuring system and method of determining particle-mass concentration in an aerosol

ABSTRACT

A particle-measuring system for determining particle mass concentrations in aerosols has a laser diode serving as a radiation source and projecting a beam of laser light through a flowing stream of the aerosol. A receiver for receiving the light from the diode after passing through the stream and converting the received light into a measurement. A frequency radiation output of the laser diode is modulated such that the frequency is substantially greater than a cutoff frequency of the receiver so that a specifiable radiation output of the laser diode is achieved on average over a duration of a measurement signal of the receiver.

FIELD OF THE INVENTION

The present invention relates to a particle-measuring system. Moreparticularly this invention concerns a method of determining aparticle-mass concentration in an aerosol.

BACKGROUND OF THE INVENTION

Such a system typically comprises a laser diode serving as radiationsource and a receiver that receives light emitted by the laser diodethrough an aerosol and converts it into a measurement. A method ofdetermining the particle mass concentrations in aerosols entailsemitting and projecting light through an aerosol with a laser diode andreceiving the light after passing through the aerosol with a receiverand a parameter of the received light into a measurement.

The laser diodes of such particle-measuring systems are not idealcomponents. Their emitted radiation output changes with the operatingtemperature and also over their lifetime. Therefore, most laser diodesused for such purposes are provided with a photo diode that isirradiated by a small portion of the radiation emitted by the laserdiodes. Accordingly, the current generated in the photo diode islinearly proportional to the output emitted by the respective laserdiode.

The change in the radiation output of the laser diodes can then becompensated for by measuring the photo diode current as equivalent tothe radiation emitted by the laser diode and calculating a correctionfactor from this that is applied to the particle-measurement signals.

Alternatively, it is possible to actively regulate the operating currentof the laser diode such that the radiation emitted by the laser diode isat a desired level. The radiation emitted by the laser diode has its setpoint at exactly one defined photo diode current. The radiation outputof the laser diode is regulated with feedback to this set point. If thephoto diode current is less than this set point, the operating currentof the laser diode is increased. If the photo diode current is greaterthan this set point, the operating current of the laser diode isreduced.

Operating mode switching represents a problem that has been hithertounsolvable. This expression denotes a characteristic of a laser diodeaccording to which, in certain operating modes, which are determined bythe operating temperature of the laser diode and the operating currentthereof, operating mode transitions from one frequency to anotherfrequency take place spontaneously. These spontaneous transitions havean unstable transition range in which chaotic, spontaneous changes occurbetween two or more operating modes.

The change from one operating mode to another operating mode isaccompanied by a change in the output of the laser diode radiationoutput. Such switching of operating modes produces laser noise, forexample in the form of identity noise and noise from other parameters.

The jumps in the emitted radiation output are in the μs range and alterthe power of the radiation that is incident on the receiver as scatteredlight. The greater the scattered light component on the receiver basedon the total emitted radiant power, the greater the alternating-voltagesignal induced by the switches in the operating modes or by the lasernoise.

In simple and compact single-particle counting photometers, which cannotbe further reduced due to the scale of the scattered light component,changes in intensity caused by these switchings of the operating modescannot be distinguished from true signals caused by particles.Accordingly, these dummy signals originating from the change between theoperating modes are interpreted as particle signals and then result in afalse particle-measurement signal, generally one that is far too high.

Particularly during operation of laser diodes on integrated circuits forautomatic power control, a strong ramping-up of power fluctuations hasbeen observed. The integrated automatic power-control circuit attemptsto correct the variation in the power output by adjusting the operatingcurrent of the laser diode. With control times in the range of a fewmilliseconds, however, conventional integrated automatic power-controlcircuits are much too sluggish or too slow. Ultimately, it is preciselythe control response of such integrated circuits that has led to thelaser diode being operated for a very long time in an operating rangewith frequently changing operating modes. Accordingly, theparticle-measurement signals provided by the particle-measuring systemwere unusable for long periods of time.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved particle-measuring system.

Another object is the provision of such an improved particle-measuringsystem and method of determining particle mass concentrations inaerosols that produce reliable and correct particle-measurement signalsin a sustained manner.

SUMMARY OF THE INVENTION

A particle-measuring system for determining particle mass concentrationsin aerosols has according to the invention a laser diode serving as aradiation source and projecting a beam of laser light through a flowingstream of the aerosol. A receiver for receiving the light from the diodeafter passing through the stream and converting the received light intoa measurement. A frequency radiation output of the laser diode ismodulated according to the invention such that the frequency issubstantially greater than a cutoff frequency of the receiver so that aspecifiable radiation output of the laser diode is achieved on averageover a duration of a measurement signal of the receiver.

In other words, the inventive object is achieved according to theinvention in that a radiation output of the laser diode can be modulatedat a frequency that is substantially greater than a cutoff frequency ofthe receiver, so that a specifiable radiation output of the laser diodeis achieved on average over the duration of a measurement signal of thereceiver. The radiation output of the laser diode is modulated at afrequency that is substantially higher than the cutoff frequency of thereceiver, so that a specifiable radiation output of the laser diode isachieved over the duration of a measurement signal of the receiver.

The invention moves away from the idea of operating a laser diode stablyat a single operating point. To wit, it is proposed according to theinvention to modulate the operating mode of the laser diode in such away that the laser diode passes through a large range of operatingstates or operating modes in a very short time in a single sensingcycle, it being ensured that a specifiable radiation output of the laserdiode is achieved on average over the duration of a measurement signalof the receiver.

The operating current of the laser diode can be advantageously modulatedin order to modulate the radiation output thereof.

In order to ensure that the duration of an operating mode no longeroccurs as an artifact downstream of the receiver, it is advantageous ifa modulation depth of the modulation of the radiation output is selectedsuch that a high number of operating modes can be passed through while asingle particle-measurement signal is generated by the receiver.

Expediently, the frequency of the modulation of the radiation output ofthe laser diode should exceed the cutoff frequency of the receiver by afactor of at least ten.

According to the invention, a particle-measuring system and a method ofoperating same is provided that can ensure that correctparticle-measurement signals are generated over the long term.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages will become morereadily apparent from the following description, reference being made tothe accompanying drawing in which:

FIG. 1 is a schematic view of a first embodiment of the invention; and

FIG. 2 is a similar schematic view of a second embodiment of theinvention.

SPECIFIC DESCRIPTION OF THE INVENTION

As seen in FIG. 1, a first embodiment of an optical particle-measuringsystem 1 according to the invention is embodied as an aerosol photometer(APM) and serves to determine the particle mass concentration in anaerosol.

The aerosol photometer 1 has a laser diode serving as monochromaticlight source 2. The light radiation emitted by the laser diode 2 of theaerosol photometer 1 is concentrated in an optical lens 3. The lightbeam leaving the optical lens 3 traverses a gas stream 4 that entrainsthe aerosol to be measured. Light is reflected from the particles of theaerosol contained in the gas stream 4 toward an additional optical lens5. The two optical lenses 3 and 5 determine the measurement volume 6shown schematically in FIG. 1. The light radiation directed at aphotodetector 7 serving as receiver due to the particles present in themeasurement volume or in the measuring chamber 6 and focused by theoptical lens 5 is detected by the photodetector 7, and a photometermeasurement corresponding to the detected light radiation is forwardedto an evaluation unit 17 of the optical particle-measuring system 1 thatis not shown in the drawing.

The photometer measurement forwarded by the photodetector 7 of theaerosol photometer 1 to the evaluation unit 17 corresponds to theparticle load present or detected in the measurement volume 6.

In the case of the aerosol photometer 1 of the embodiment described withreference to FIG. 1, a great advantage resides in the fact that themeasurement detected in the evaluation unit 17 is independent of theflow rate of the gas stream 4 carrying the aerosol to be measured. Inthe case of the aerosol photometer 1, the measurement volume isdetermined by the optical measurement volume.

An embodiment of the optical particle-measuring system shown in FIG. 2has a single-particle counting photometer 10. The single-particlecounting photometer 10 also has a laser diode serving as monochromaticlight source 11. The laser diode 11 emits light radiation that isfocused by an optical lens 12. The light beam that is focused in theoptical lens 12 traverses a gas stream that carries the aerosol to bemeasured. A measurement volume 14 of the single-particle countingphotometer 10 is substantially smaller than the measurement volume ofthe aerosol photometer 1. In the embodiment of the single-particlecounting photometer 10 of FIG. 2, this is achieved in that the lightemitted by the laser diode 11 is focused much more intensely by theoptical lens 12 than is achieved by the optical lens 3 of the aerosolphotometer 12. The measurement volume or measuring chamber 14 of thesingle-particle counting photometer 10 is dimensioned in considerationof the expected values of aerosols to be measured, so that only a singleparticle of the aerosol is present therein. The light radiationreflected in the measurement volume 14 of the single-particle countingphotometer 10 is directed through an optical lens 15 at photodetectorserving as a receiver of the single-particle counting photometer 10 thatis located in the radiation path behind the optical lens 15. For eachindividual particle of the aerosol that travels through the measurementvolume or measuring chamber 14 of the single-particle countingphotometer 10 with the gas stream 13, a single measurement correspondingto a single particle is thus forwarded at the photodetector 16 of thesingle-particle counting photometer 10 to an evaluation unit 17 of thesingle-particle counting photometer 10. Each individual measurementcorresponds to the light reflected by a single particle of the aerosolto be measured and directed through the optical lens 15 at thephotodetector 16 of the single-particle counting photometer 10.

In contrast to the aerosol photometer 1 described above in connectionwith FIG. 1, the single-particle counting photometer 10 detectsindividual particles. Such single-particle counting photometers 10 areused to measure comparatively low particle concentrations, for examplein interior spaces. Within the scope of their area of application, i.e.at comparatively low to medium particle concentrations usually between1000 and 20,000 particles/cm³, high-quality information can be obtainedabout the particle count and the particle size distribution in theaerosol.

The laser diode 2 or 11 of the two above-described particle-measuringsystems 1, 10 is not operated or driven at a single, predeterminedoperating point in a stable operation. Rather, in the case of the laserdiodes 2, 11 of the particle-measuring systems 1, 10, the operatingcurrent of the laser diodes 2, 11 is modulated by a driver 18 such thatthe laser diodes 2, 11 pass through a wide range of different operatingconditions in a very short time.

The modulation depth is selected such that a very large number ofoperating states or operating modes are passed through, for example 100operating modes. Due to the high modulation frequency, the time spent ina single operating mode is much shorter than the time required for thegeneration of a single particle-measurement signal of the receiver 7,16.

For example, the modulation frequency can be set up as follows:

The receivers 7, 16 have a cutoff frequency of about 100 kHz. In orderto ensure that no appreciable artifacts of the modulation of thelaser-diode operation appear in the particle-measurement signal, it isspecified that the frequency of the modulation of the operation of thelaser diodes 2, 11 is at least ten times the amplifier cutoff frequencyof the photodetectors 7, 16. If ten operating modes are passed throughduring one modulation period due to the modulation depths of the laserdiodes 2, 11, the shortest time interval of the jumps between theoperating modes is 100 kHz×10×10×2=20 MHZ. The duration of an operatingmode is thus 0.05 μs, and a single operating mode no longer occurs as anartifact behind the photodetectors 7, 16.

I claim:
 1. A particle-measuring system for determining particle massconcentrations in aerosols, the system comprising: a laser diode servingas a radiation source and projecting a beam of laser light through aflowing stream of the aerosol; a receiver for receiving the light fromthe diode after passing through the stream and converting the receivedlight into a measurement; and means for modulating a frequency radiationoutput of the laser diode such that the frequency is substantiallygreater than a cutoff frequency of the receiver so that a specifiableradiation output of the laser diode is achieved on average over aduration of a measurement signal of the receiver.
 2. Theparticle-measuring system defined in claim 1, wherein an operatingcurrent of the laser diode is modulated by the means in order tomodulate the radiation output thereof.
 3. The particle-measuring systemdefined in claim
 1. wherein a modulation depth of the modulation of theradiation output is selected such that a high number of operating modescan be passed through and that a single particle-measurement signal isgenerated by the receiver.
 4. The particle-measuring system defined inclaim 1, wherein the frequency of the modulation of the radiation outputof the laser diode exceeds the cutoff frequency of the receiver by afactor of at least ten.
 5. A method of determining particle massconcentrations in aerosols, the method comprising the steps of:projecting light through an aerosol from a laser diode; receiving theprojected light after projection through the aerosol in a receiver andconverted the received light into a measurement; and modulating aradiation output of the laser diode at a frequency that is substantiallyhigher than a cutoff frequency of the receiver so that a specifiableradiation output of the laser diode is achieved over a duration of ameasurement signal of the receiver.
 6. The method defined in claim 5,wherein the radiation output is modulated by modulating an operatingcurrent of the laser diode.
 7. The method defined in claim 5, furthercomprising the step of: passing the radiation output of the laser diodethrough a high number of operating modes while the receiver generates asingle measurement for a particle signal.
 8. The method defined in claim5, wherein the frequency of the modulation of the radiation output ofthe laser diode exceeds the cutoff frequency of the receiver by a factorof at least ten.